Interpretive Summary: Cytoplasmic male sterility (cms) is of critical importance in the production of hybrid grain sorghum. Many sources of cms in sorghum are available, but only one source is used in the U.S. Other sources found provide increased genetic diversity, but are not well understood and thus are not used. We worked with a unique source of cms, the IS1112C line, where we have established the likely cause of cms and a likely mechanism by which fertility is restored in hybrids. This peculiar type of cms requires that a defect must appear in the pollen grains. The required effects, transcription of a mitochondrial gene, RNA editing of the gene, and destruction of part of the mRNA of the gene, were documented in leaves. We have extended these data, and have found that each effect also occurs in developing pollen grains. These observations thus confirm our earlier conclusions about this system, and increase the degree of certainty of knowledge of the IS1112C source of cms.

Technical Abstract:
The restoration of male fertility in the A3, IS1112C source of cytoplasmic male sterility (cms) in sorghum is exacted in a gametophytic manner. One required nuclear gene, Rf3, regulates a nucleolytic transcript processing activity, cleaving sequences internal to the chimeric mitochondrial open reading frame orf107. We examined mitochondrial transcription and RNA editing, and action of Rf3, in developing pollen from a male-sterile line, the progenitor, male-fertile line, and the fertile F1 to determine if these expression processes were manifested at the haploid, pollen stage. Steady-state levels of orf107 transcripts, and nucleolytic processing conferred by lines carrying Rf3, were indistinguishable from leaf tissues, indicating comparable expression in pollen. The editing frequency of two orf107 editing sites in male-sterile plants was higher than that of male-fertile plants, but the transcripts were generally less edited than in leaf tissues. A differential editing effect was manifested by near complete editing of an orf107 site shared with atp9, indicating that factors influencing editing frequency of the chimeric transcript are independently regulated. Thus mtDNA transcription and RNA editing, and action of the nuclear gene Rf3, are expressed at the haploid stage, consistent with characteristics of fertility restoration in this gametophytic system.